Recently, it was shown that the film morphology of an active layer in bulk heterojunction solar cells, made with a blend of an electron donor material and an electron acceptor material, is of great importance for solar cell power efficiency. Yang et al showed that the blend Poly(3-hexylthiophene) (P36T)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has a different nano-morphology if a post-heat treatment step is applied to the active layer (after its deposition). Post-heat treatment of the active layer brings about a heterogeneous morphology with polymer-rich and PCBM-rich zones (X. Yang et al, Nanoletters, 2005, vol.5, p. 579). Recently, a new method for the preparation of active layers of polymeric solar cells, without the need for thermal post-treatment, was disclosed (S. Berson et al, WO 2007/048909 also, Adv. Funct. Mater, vol. 17, iss. 8, p. 1377-1384). Poly(3-hexylthiophene) (P36T) nano-fibres were prepared as a highly concentrated dispersion in a solution of low molecular weight P36T. The crystalline nanofibre concentration in the dispersions was strongly dependent on the molecular weight of the starting polymer as only the high molecular weight fraction forms nano-fibers, (the low molecular weight polymer fraction stays in solution).
WO2007/048909 discloses a process for preparing a polymeric composition having photovoltaic properties comprising a step of mixing in a solvent at least one electron donating polymeric semiconductor substantially in the form of nanofibres and an electron accepting material in the solvent, said nanofibres representing at least 10% in weight of the electron-donating polymeric semiconductor material. However, the control of the nano-fiber content using this method is very tedious and labour-intensive as it involves a large number of steps.